The collimator slit is generally narrow and of equal width. An exception is a V-shaped slit as disclosed in the Finnish Patent Publication No. 54856. However, it is preferable in various applications of radiography that the width and/or shape of the slit could be changed, case by case by means of presetting or by continuous adjustment during radiography. A problem is, however, to effect the adjustment accurately and symmetrically on both edges of the slit in order to maintain accurate concentration and alignment of the beam of rays.